This invention pertains generally to radar antennas and particularly to radar antennas fabricated using microstrip techniques.
The need for high gain, light weight, and low cost antennas for use in guided missiles (referred to hereinafter as "missiles") has led to the development of antenna elements made using microstrip techniques. Antenna elements so made are easily fabricated and are well adapted to use in missiles. Thus, for example, a microstrip antenna element in the form of a circular disk is described in an article by L. C. Shen, S. A. Long, M. R. Allerding and M. D. Walton entitled "Resonant Frequency of a Circular Disc Printed Circuit Antenna," Vol. AP-25, pages 595-596, July 1977; microstrip antenna elements in the form of rectangular patches are described in an article by R. E. Munson entitled "Conformal Microstrip Antennas and Microstrip Phased Arrays," Vol. AP-22, pages 74-78, January 1974; and microstrip antenna elements in the form of a circular ring are described in an article by J. W. Mink entitled "Circular Ring Microstrip Antenna Elements," IEEE-APS International Symposium Digest, pages 605-608, 1980; or in an article by I. J. Bahl and S. S. Stuckly entitled "Characteristics of Microstrip Ring Antennas," IEEE-APS Symposium Digest, Vol. I, pages 27-30, 1981.
All of the microstrip antenna elements described in the cited articles have co-planar feeds, meaning that the feeds are formed on the same surface as the microstrip antenna elements themselves. It follows, then, that in the design of an antenna using any such elements it is not possible to utilize the entire area of an antenna aperture for the microstrip antenna elements because an appreciable portion of such area must be covered by the feeds.
Microstrip antenna elements reactively coupled to feeds to avoid the shortcomings of co-planar elements and feeds are described in U.S. Pat. No. 4,054,874. The antenna elements are dipoles and the feeds are disposed in the dielectric medium between the dipoles and the ground plane of the microstrip. Circular polarization may be provided by forming orthogonally disposed pairs of dipoles and separately feeding the dipoles in each pair. The bandwidth for either linearly polarized or circularly polarized microstrip antenna elements is, however, rather narrow, thereby counterbalancing any advantage gained from the reactive feed.